Such a method, and a device suitable for it, are known according to the present generic type from EP 0 711 627 B1.
A number of methods are known from the prior art with which tear lines can be produced by means of lasers in vehicle interior trim parts with an integrated airbag. They differ, among other respects, by the different measures that are taken to create a tear line that is first of all, not perceptible from the exterior of the vehicle interior trim part and, second, has an optimal, defined and reproducible tearing resistance along its length.
According to the two publications below, these two problems are solved by performing a material ablation along the tear line, with a measurement value relevant to the residual wall thickness being determined and the laser being controlled such that a defined residual material thickness is preserved underneath the ablation volume.
According to EP 0 827 802 B1, a residual wall thickness (material thickness) is assured by detecting the transmitted radiation from the exterior with a sensor. For this purpose, the sensor must be arranged on the optical axis of the laser directed at the vehicle interior. No information is given for realizing the relative movement between the laser beam and the vehicle interior trim part necessary for producing the tear line. In practice, when using a laser cutting system, such as VOTAN A manufactured by JENOPTIK AG of Jena, Germany, to perform the method of this patent, the vehicle interior trim part is moved and the laser and sensor remain stationary.
The laser beam and the sensor could also be moved relative to the vehicle interior trim part. In order to ensure a stable position of the laser beam, it would be necessary to create a mechanically stable connection between a robot arm guiding the laser beam and the sensor, which would leave only a limited range of movement. Movement of the laser beam by means of a scanner would require an additional drive unit to synchronize the movement of the deflection elements of the scanner with the movement of the scanner.
These difficulties do not exist in a solution according to EP 0 711 627 B1, in which a sensor is used that covers the work field of the laser beam and therefore need not be moved along with it, but can instead remain stationary.
In an embodiment for a method and a device described in the above document, the vehicle interior trim part is held on an ultrasound sensor and the laser beam is guided along the tear line, the ablation being performed in the form of a continuous groove or a series of perforations (blind holes). The ultrasound sensor detects signals corresponding to the thickness of the material and thus supplies a feedback signal to a central computer in order to modify the position of the laser generator and/or its output power and thus control the thickness of the material remaining after the notching.
A solution according to EP 0 711 627 B1 is advantageous compared to the solution according to EP 0 827 802 B1 to the extent that the sensor and the vehicle interior trim part are held stationary. The movement of only the laser beam allows, if necessary, a higher positioning speed between the laser beam and the respective machining site on the tear line. The unrestricted functioning of an extensive ultrasound sensor according to EP 0 711 627 B1 is doubtful, however. A prerequisite would be that the sensor surface be in air-free contact with the surface of the vehicle interior trim part. This becomes difficult particularly if the surface of the vehicle interior trim part has a three-dimensionally extended free-form surface or even if its surface has a structured height profile.